Thermal hydrolysis and steam explosion is a known method to break down biomass so that it is better suited to anaerobic degradation. There are several patented methods that describe both batch wise and continuous processes. Batch wise treatment of biomass leads to both approved sterilisation and infection prevention by ensuring the necessary residence time at high temperatures. This is different to continuous processes that can not document that all the biomass has been at the right temperature for sufficient time. There are, in particular, two methods for batch wise thermal hydrolysis that are known.
WO96/09882 (Solheim et al) describes an energy efficient process where the biomass is preheated in a preheating tank with the help of steam returned from a downstream process before it is pumped into one of several parallel reactors. The biomass is supplied new steam for heating and is held at the predetermined temperature and pressure for the required time. Thereafter, the reactor is relieved and steam is led back to the reheating tank for energy recovery. When the reactor pressure has sunk to a desired level (typically 2-4 bara) the steam return from the reactor to the preheating tank is shut. The biomass is thereafter blown over to a pressure relief tank at a low pressure (typically 1.2 bara). The advantage of this system is that with a relatively low pressure in the pressure relief tank one can recover a large fraction of the energy from the reactor after completed thermal hydrolysis. The pressure reduction in the reactor before it is blown to the pressure relief tank is described as necessary to reduce the erosion problems in the pipe system between the reactor and the pressure relief tank.
U.S. Pat. No. 6,966,989 (Højsgaard et al) does not use, like WO96/09882, a preheating tank but has parallel reactors that also function as preheating tanks. This is achieved in that one reactor at a high pressure is relieved by letting steam over to a waiting reactor at low pressure. When these reactors are at the same pressure, the steam transfer is stopped and the hydrolysed biomass in the reactor is blown over to a low pressure pressure relief tank. Energy recovery from the pressure relief tank is not described. By filling a reactor with steam from another reactor one does not come down very far in pressure and the energy recovery is thereby not optimal.
Common for these two systems is a limited possibility to blow the biomass from the reactor to the pressure relief tank at high pressure. They have both a controlled pressure relief of the steam phase from the reactor before the biomass is blown over into a pressure relief tank. This occurs at a considerably lower pressure than the hydrolysis pressure.
An element that makes the two systems more expensive is that the reactors must have both a steam line with a control valve on top and a liquid/slurry line with a shut-off valve in the bottom of the reactor.
Furthermore, a method and device is known from WO0073221 (Solheim et al) for continuous hydrolysis of the biomass/sludge. The biomass is preheated in a tank with spent steam from the pressure relief tank of the process. The heated sludge is fed via a feeding pump and a mixing unit to a reactor. The pressure to the sludge is increased before this is fed into the reactor. In the reactor the temperature and pressure are controlled and fresh steam can be supplied to the reactor. The heated and pressurised biomass is then fed into a pressure relief tank which is fitted with a nozzle for the relief of the pressure. In this tank, a sudden reduction of the pressure occurs, a so-called steam explosion. Steam from the pressure relief tank can be returned to the preheating unit.
A method for thermal hydrolysis of lignocellulose is known from WO 2008026932 (Solheim et al). The lignocellulose is hydrolysed with the use of two reactors, whereupon these are supplied steam for heating and pressurising. The reactors can operate sequentially before the mass is fed in to a flash tank, where, because of the pressure difference between them, a steam explosion of the mass occurs.
Another limitation with the prior processes is the time it takes for the pressure relief of the tank before one blows the biomass to the pressure relief tank. It takes time to boil off the steam for the pressure relief. This is described in U.S. Pat. No. 6,966,989 with a typical cycle time of 100-360 minutes, with a preferred cycle time of 150-160 minutes with the use of three reactors.
Since the steam shall exit at the top of the reactor without the biomass following, the reactor must have a considerable steam volume above the liquid phase to avoid foaming and liquid being dragged out in the steam return line. This limits the active volume of the reactor. A further limitation is the need for a large liquid surface to prevent shock boiling. This results in a need for a large diameter/height ratio in the reactor, which is not optimal for a pressure tank.
Steam explosion of biomass at large pressure drops over a short time will give greater cell destruction and larger bioavailability of the biomass. This will lead to a higher production of biogas in a downstream, anaerobic degradation tank. Therefore, it is desirable with the fastest possible and the biggest possible pressure drop when the biomass is blown over into the pressure relief tank.
The system according to the present invention protects the possibility of blowing the biomass at hydrolysis pressure, the highest in the process. This is done by using an especially robust nozzle at the end of the blow pipe between the reactor and the pressure relief tank which takes nearly the whole of the drop in pressure. Thereby the largest possible steam explosion and cell destruction in the biomass are ensured. The nozzle consists of an erosion resistant material, preferably a ceramic material, and is formed so that the biomass is blown down onto the liquid surface in the pressure relief tank. The erosion is thereby reduced to a minimum. The velocity upstream of the nozzle is low and ordinary pipe parts can thereby be used in the blow pipe between the reactor and the pressure relief tank.
As one blows at the hydrolysis pressure and has no pressure relief of steam at the top of the reactor, one can fill the reactors nearly to the top. The only limitation is to have sufficient volume for the condensing steam which is used for the heating (typically about 10% extra volume).
The reactors in the system according to the present invention can be manufactured to be long and slim, as there are no requirements to the surface area of the liquid phase because no steam is to be boiled off for pressure relief. This is cost effective with regard to use of materials and means a reduced need for floor area.
The system and the method according to the present invention optimise the cycle time to utilise the reactor volume optimally. Normally the requirement for sterilisation will be to keep the biomass at 133° C. for at least 20 minutes. When the system comprises, for example, three reactors, filling for 15 minutes, holding for 20 minutes and emptying for 15 minutes will lead to a constant consumption of steam while the requirement for sterilisation is met. This cycle time of 45 minutes means a considerably increased capacity compared to, for example, the system according to U.S. Pat. No. 6,966,989. If there is no requirement for sterilisation, the holding time and the temperature can be different from those given above. An embodiment of the system according to the present invention with three reactors can, with this cycle time, fill and empty four reactors per hour (see FIG. 2).
The system according to the present invention encompasses at least two pressure relief tanks in series to recover more energy. The second pressure relief tank is under vacuum so that steam boils off at lower temperature (for example, 0.7 bar gives 90° C.). This results in a typically 20% better energy efficiency than the system according to WO96/09882.
By controlling the flow of energy to the reactor and/or preheating tank, respectively, one can exactly achieve the desired pressure and temperature in the preheating tank. This results in a flexible system with good process control. With this control one is flexible with regard to further preheating of the biomass even before it gets to the preheating tank. By leading more of the energy to the reactor, one can permit preheating of the sludge with low temperature energy.
By using two pressure relief tanks the pressure in the first can be higher than that described in Solheim et al without affecting the total energy recovery. This can be advantageous because the preheating tank will thereby reach a higher pressure. All non-condensing gases in the process (foul gas) end up in the preheating tank. These gases often smell badly and ought to be transported in a closed system to the decomposition tank for biological degradation. If the pressure in the preheating tank is low, it may be necessary with a system that compresses the non-condensing gases to transport them into the degradation tank. With the present invention one eliminates the need for such a system.
Neither WO96/09882 nor U.S. Pat. No. 6,966,989 describe handling of the non-condensing gases. In the system according to U.S. Pat. No. 6,966,989 these gases can be accumulated in the reactor as flash steam only goes between the reactors. This represents a considerable problem as the pressure and the temperature will thereby no longer follow the saturation curve for water/steam. The pressure in the reactors can rise to a nominally correct level without the necessary hydrolysis temperature being reached.
A possible way to eliminate accumulation of non-condensing gases is to ensure that the reactor is emptied completely for the liquid phase and thereafter also empty the steam phase from the reactor to the pressure relief tank. This is time consuming and reduces the capacity of the system. The present invention solves this problem by leading the small amounts of non-condensing gases from the reactor to the preheating tank in a small gas return line. This line is opened a short period during the filling of the reactor with biomass.
An important feature that distinguishes the present invention from what is known from WO0073221 is that the method according to the present invention is a batch process while the method according to WO0073221 is a continuous process. Correspondingly, the methods according to WO96/09882 and WO2008026932 are also batch processes. The main feature of the present invention is not direct steam supply to the reactor, but the fact that with the help of a nozzle one can treat sludge that contains erosive particles. This is not possible with the method according to WO0073221 as this uses a pump to pressurise the sludge in the reactor. Neither WO96/09882 nor WO2008026932 comprise a nozzle and must therefore pressure relieve the reactor before the sludge can be transferred to the pressure relief tank. Thereby, the present invention solves the problems which WO96/09882 and WO2008026932 have not solved.
The aim of the present invention is to provide a system and a method with which the above mentioned disadvantages of the previously known solutions are eliminated or much reduced.
This is achieved with a method for thermal hydrolysis and steam explosion of biomass, said method encompasses the following steps:                (i) Feeding the biomass approximately continuously into a first preheating step and heating the biomass,        (ii) leading the preheated biomass sequentially into at least two reactors,        (iii) heating and pressurising the reactor by the supply of steam,        (iv) holding the reactor(s) at a given temperature and pressure over a given time,        (v) leading the heated and pressurised biomass from the reactor(s) to a first pressure relief tank without any substantial reduction in pressure and relieving the pressure of the biomass quickly with the help of a nozzle so that the biomass is broken up,        (vi) leading the biomass from the first pressure relief tank to a second pressure relief tank at a lower pressure than the pressure in the first pressure relief tank,        (vii) leading the treated biomass to a downstream installation for further treatment.        
The pressure reduction in step (v) is preferably in the area 2-13 bar and the pressure reduction in step (vi) is preferably in the area 0.4-1.6 bar.
The pressure in the reactor is preferably of the order of 4-14 bar abs.
The pressure in the first pressure relief tank is preferably of the order of 1.2-2 bar abs.
The pressure in the second pressure relief tank is preferably of the order of 0.3-0.8 bar abs.
The number of sequential reactors is preferably three and step (ii) has a duration of about 15 minutes, step (iv) has a duration of preferably about 20 minutes and step (v) has a duration of preferably about 10 minutes.
The method is further characterised by stopping the filling of the first reactor and immediately starting the filling of the second reactor and when the second reactor is filled, starting the filling of the third reactor and repeating this sequence continuously.
Steam from the first pressure relief tank is led to the preheating tank to heat this.
Steam from the second pressure relief tank is led to the preheating tank and/or the reactor for the heating of these.
Non-condensing gases are led back from the reactor to the preheating tank and thereafter the non-condensing gases are led from the preheating tank to a subsequent treatment step.
The invention also relates to a device for thermal hydrolysis of organic material, said device encompasses a preheating tank for preheating of the biomass to a desired temperature, one or more reactors connected with the preheating tank and one or more pressure relief tanks connected with the reactor(s) for relief of the pressure from the biomass, said device is characterised in that it comprises a steam supply line that supplies steam from a steam supply source to the, at least, two reactors, a line that leads pressurised biomass from the reactor(s) to a nozzle in the first pressure relief tank without any substantial pressure reduction for relief of the pressure of the biomass from the reactor and a line for supply of steam from the one pressure relief tank to the preheating tank, and a second pressure relief tank connected with the first pressure relief tank for relief of the pressure of the biomass from the first pressure relief tank, said second pressure relief tank being connected with the preheating tank with a line for return of steam from the second pressure relief tank to the preheating tank.
The preheating tank is preferably connected with an agitation pump cycle for recirculation of preheated biomass, that the steam return line from the second pressure relief tank is connected with an agitation pump cycle via an ejector and/or that the steam return line from the second pressure relief tank is connected via an ejector with the steam supply line for supply of steam to the reactor(s).
The steam return line from the second pressure relief tank can possibly be connected with one or more compressors to compress the steam from the second pressure relief tank and lead the compressed steam to the preheating tank and/or the reactor(s).
The, at least, one reactor is connected to the preheating tank with a line for non-condensing gases and the preheating tank comprises a line to lead the non-condensing gases to a downstream treatment installation.
The first pressure relief tank preferably comprises one or more nozzles for the supply of hydrolysed biomass from the, at least, one reactor to achieve maximum pressure relief and disintegration of the biomass.